In wireless communication systems, such as, for example, Orthogonal Frequency Division Multiple Access (OFDMA) systems, interference diversity is an important aspect of system performance. In other words, the ability to measure interference accurately and take advantage of the interference level information is critical to proper system performance. However, when “small” messages (e.g., ACK/NAK, power control, interference control bits, single-bit messages, etc.) are transmitted in these systems, the messages provide very limited samples for interference realization, which leads to increased difficulty in the interference estimation process. Furthermore, these samples may have different interference power levels, which can severely impact the ability of a receiver to detect the single-bit or other small messages involved.
An illustrative example of such a problem is the Reverse link Acknowledgment Channel (R-ACKCH) in the current 3rd Generation Partnership Project 2 (3GPP2) joint framework proposal for the physical layer of the air interface evolution phase 2. Essentially, the R-ACKCH is a portion of a reverse OFDMA channel used for transmitting acknowledgment messages from an Access Terminal (e.g., mobile station, mobile terminal, wireless terminal, etc.) to one or more Access Networks in response to data received on a Forward Packet Data Channel. More precisely, the R-ACKCH is used to acknowledge Forward Link PHY frames transmitted on the Forward Data Channel. In the currently proposed 3GPP2 R-ACKCH design, the R-ACKCH is to be transmitted over 4 frequency tiles.
FIG. 1 is a pictorial diagram depicting the current design 100 of the R-ACKCH for the physical layer of the air interface in the 3GPP2 joint framework proposal. As shown, each R-ACKCH is being transmitted over a tile 102, 104, 106, and each tile 102, 104, 106 is one of four possible tiles. Also, two example tiles 108, 110 are shown. Tile 108 represents a tile that includes only data and pilot signal. Tile 110 represents a tile that includes data and pilot signal in the upper half-tile 110a, and R-ACKCH data in the lower half-tile 110b. Each lower-half tile 110b is further sub-divided into 4 sub-tiles 112, 114, 116, 118. Each sub-tile 112, 114, 116, 118 spans over 8 sub-carriers and 2 OFDM symbols. Each ACK bit to be transmitted will be Discrete Fourier Transform (DFT) pre-coded, and mapped into one of the 4 sub-tiles 112, 114, 116, 118. This process is repeated in each of the four possible tiles so that 4th order frequency diversity can be achieved.
In that regard, FIG. 2 is a block diagram depicting the existing transmission processing scheme 200 for the R-ACKCH. First, the ACK bits to be transmitted will be processed through a DFT pre-coder 202. Next, the output of the DFT pre-coder 202 will be input to a mapper 204, which maps the DFT pre-coder output to a sub-tile. The mapped sub-tile 206 is then placed within an ACK tile 208. Note that FIG. 2 only shows the transmission processing scheme for one sub-tile. However, each ACK bit will be transmitted in 4 sub-tiles, and each of the 4 sub-tiles will be located in a different ACK tile. Therefore, the transmission processing scheme 200 shown in FIG. 2 will have to be performed 4 times, or once for each tile. Also, note that the ACK bits that are processed through one DFT pre-coder may or may not be transmitted by the same mobile station.
For example, from the viewpoint of a particular mobile station, i, when an ACK i is transmitted, the ACK i will be transmitted in 4 sub-tiles, with each one using a possibly different DFT pre-coding index. An example of this process is illustrated by the R-ACKCH channel structure 300 shown in FIG. 3. As shown, DFT pre-coding sequence i1 304a is used for transmitting ACK i in sub-tile 1 306a, DFT pre-coding sequence i2 304b is used for transmitting ACK i in sub-tile 2 306b, and so on. After the DFT pre-coder output is mapped into a sub-tile, each sub-tile is placed into 1 of the 4 possible sub-tile positions in an ACK tile. For example, sub-tile 1 306a is placed into ACK tile 1 308a, and sub-tile 2 306b is placed into ACK tile 2 308b. Each ACK tile is then hopped 310 over frequency and multiplexed with other reverse channels to maximize frequency diversity and minimize the possibility of collision with other ACK tiles. All of the ACK tiles are then processed through an Inverse Fast Fourier Transformer (IFFT) 312 and a parallel-to-series converter 314.
Notwithstanding the numerous advantages of the existing R-ACKCH transmission scheme and channel structure, it is relatively difficult to implement suitable interference estimation for each sub-tile. For example, in order to estimate interference accurately, 8 out of 16 DFT codes for each sub-tile are to be reserved for interference estimation, which will result in a significant waste of processing resources. Additionally, if the R-ACKCH tiles in different sectors collide, the interference within a tile may not be constant, which will create even more challenges in designing for interference estimation and ACK detection. Therefore, a pressing need exists for a transmission scheme and channel structure that can improve the interference distribution, in order to facilitate interference estimation and enhance the detection of the small messages involved.